The invention relates to analysis of gases, and more particularly it relates to a method of analysis of organic compounds in chromatography and to apparatus capable of performing this method.
1. Field of the Invention
The invention can be used in chemical, food and perfumery industries, and also for analytical purposes for identifying trace quantities of amines, hydrazines (diamines) and their derivatives in analyzed mixtures.
The invention can be employed for identifying the substances defining the nature of a flavor or odor, for detecting amines emitted by living organisms, for indicating trace quantities of toxic amines in gaseous emissions of various polymeric materials, for identifying amines and their derivatives in the products of pyrolysis of microorganisms.
2. Description of the Related Art
Techniques of gas chromatography are broadly employed nowadays for identifying nitrogen-containing organic compounds with the use of ionization detectors offering the highest response among available detectors, e.g. coulometer detectors, photo-ionization detectors, etc. Any technique employed presents its own problems related to both the selectivity and sensitivity in registration of components of an analyzed mixture. The most sensitive to nitrogen-containing organic compounds among the mentioned ionization detectors is the thermal ionization detector whose threshold response is 10.sup.-12 . . . 10.sup.-13 g/s; however, it has no selectivity with respect to amine compounds and ammonia.
The most efficient techniques of high-response detection of amines, hydrazines and their derivatives are those based on the phenomenon of ionization of molecules in the course of their thermal desorption from a surface.
It has been found that the best materials for thermal emitters of ions, or thermoemitters are oxidized tungsten, molybdenum, nickel, chromium, iridium and several other metals.
There is known a method of analysis of organic compounds by the method of surface ionization (SU, A, 439747), including the steps of feeding vapors of an analyzed mixture onto a thermoemitter under reduced pressure, and measuring the current of described ions at the collector.
However, it is commonly known that the interaction of organic molecules with the oxide layer of the surface of a thermoemitter gives rise to reduction of the oxides by the products of decomposition of the compounds being analyzed. This leads to varying catalytical and thermal emission characteristics of the surface, and, consequently, to the reduction of the ionization current--the so-called "poisoning" of the thermoemitter. To stabilize the thermal emission characteristics of the surface, it is necessary to feed in an auxiliary gas, e.g. oxygen or air, jointly with the analyzed substance. This condition is automatically satisfied in surface-ionization detectors employed in gas analyzers of atmospheric air. When a detector, however, is connected to a chromatographic column, its normal operation requires the supply of either oxygen or air in quantities ensuring that within the range of working temperatures of the thermoemitter the rate of oxidation of the surface would be not lower than the rate of reduction of its oxides by the products of decomposition of organic compounds.
The closest prior art of the disclosed method by the technical essence and attained effect is the method of measuring ultra-trace admixtures of organic substances with the use of a surface-ionization detector (SU, A, 728067). The analyzed mixture separated in the chromatographic column is directed into a detector jointly with an auxiliary gas, the working temperature of the thermoemitter and the quantity of the auxiliary gas being selected so as to maintain permanence of the surface ionization factor of the analyzed substance, and the ionization current is measured.
However, when the gases are supplied in this manner, the analyzed mixture leaving the chromatographic column is pre-mixed with the auxialiary gas and only then is directed to the ionizing surface of the thermoemitter. The presence of an auxiliary volume required for the pre-mixing results in some partial re-mixing of the separated components of the mixture, which adversely affects the accuracy of measuring the registered components by the method of the prior art. Furthermore, with the flow rates of the carrier gas associated with gas chromatography, adsorption of a part of the analyzed substance on the walls of the passages leading to the detector results in reduced intensity of chromatography peaks, and, hence, to a reduced threshold sensitivity of the analysis.
The known detectors of organic compounds whose operation is based on ionization of organic compounds on the surface of heated solid bodies are made of a diode of which the cathode serves as the collector and the anode serves as the emitter of positive ions. The ionization efficiency and, consequently, the sensitivity of surface-ionization detectors is dependent on their geometry, i.e. on the shape and relative positions of the collector and emitter, and also on the material of the emitter. The ionization efficiency E of a detector is expressed as EQU E=.gamma..multidot..beta.,
where
.gamma. is the factor of engagement of the substance, dependent on the design of the detector and expressed as a ratio of the number of molecules of the analyzed substance engaging the surface of the emitter to the total number of molecules of the analyzed substances passing through the detector; and PA0 .beta. is the factor of surface ionization of ionizable particles, equalling a ratio of the number of desorbed ions to the number of analyzed molecules engaging the surface of the emitter, dependent on the thermal emission and catalytical properties of the emitter. PA0 the relatively great volume of the detector, which could not be reduced in principle; the thermoemitter should be beyond the confines of flame when the detector is operated in the flame ionization detector mode, whereas in the operation in the surface-ionization detector mode the thermoemitter should be as close as possible to the nozzle detector. Therefore, the disclosed design embodies a compromise settlement which would not offer optimized relationships for operation with utmost sensitivity; PA0 in the surface-ionization detector mode, the components of the separated mixture pre-mixed with the oxidizing (auxiliary) gas has to pass in this known apparatus a certain path in the ionization chamber prior to reaching the ionizing surface of the thermoemitter. This leads to re-mixing of the separated components of the mixture, impairing the accuracy of the analysis.
There is known a detector for gas chromatography (SU, A, 179509), including a housing accommodating therein a cathode serving as the collector, receiving coaxially therein a heated anode serving as the emitter, shaped as a cylinder, and connections for feeding and delivering the analyzed gas. The end face surface of the anode is arranged to face the inlet passage of the analyzed gas, parallel to the end face surface of the cathode with which it defines a flat-parallel gap.
A detector of this design has its operability and characteristics essentially dependent on the linear dimensions and ratios of its structural components, particularly on the extent of the gap between the cathode and anode. Optimum ratios have to be selected experimentally, in a trial-and-error manner.
Furthermore, this design prohibits independent supply into the detector of the auxiliary and analyzed gases, which adversely affects the performance of the detector.
There is further known an apparatus for gas chromatography (International Application JP WO 86/06836, Int.Cl..sup.4 G 01 N 27/62, 30/64, 20.11.86), comprising a combination of a surface-ionization detector and a flame ionization detector.
The apparatus includes an ionization chamber connected with the chromatographic column through means for feeding the analyzed gas, terminating in a quartz nozzle. The ionization chamber accommodates a heated electrode (the thermoemitter) facing the nozzle, a nozzle electrode mounted on the nozzle, a cylindrical electrode (the collector) enclosing the heated electrode (the emitter), adjoining the nozzle electrode, the corresponding power supply means. Furthermore, the ionization chamber is provided with an opening for feeding in either oxygen or air, and the means for feeding the analyzed gas communicates through cut-off valves with a source of either oxygen or air, and with a source of hydrogen.
Among the shortcomings of this known apparatus are:
Furthermore, the relatively great volume of the ionozation chamber of the apparatus would not allow the operation with capillary microcolumns, although such operation combines small volumes with high sensitivity.
The closest prior art of the disclosed apparatus by its technical essence and attained effect is the apparatus for analysis of organic substances (Zandberg E.Ya. et al., "Vysokochuvstvitel'nyi detektor aminov i ikh proizvodnykh /High-sensitivity detector of amines and their derivatives/", Zhurnal Analiticheskoi Khimii, p. 1188, 1980, Ser. 6, Vol. 35), wherein the surface-ionization detector communicates via a feed connection with the outlet of the chromatographic column. The surface-ionization detector includes a housing accommodating a cylindrical collector coaxially receiving therein an incandescent thermoemitter in the form of a coil of oxidized molybdenum wire with connecting current leads; the housing having mounted therein connections for feed and delivery of the analyzed gas. The relative positions and ratios of the dimensions of the structural components of the detector provide for swirling motion of the flow of the analyzed substance. The apparatus of the prior art offers the additional advantages of a reduced gas volume, substantial reduction of the rate of flow of the analyzed substance, enhanced temperature distribution lengthwise of the themoemitter coil, which, in its turn, improves the ionization conditions.
However, the apparatus of the prior art would not ensure sufficient accuracy and sensitivity of analysis of organic compounds on account of its not being suited for independent supply into the detector of the auxiliary gas and separated components of the mixture under analysis. The mixing of auxiliary and analyzed gases takes place within the volume between the feed connection and the connection for supplying the auxiliary gas, so that the separated components of the analyzed mixture partly re-mix within this volume upstream of the entrance of the detector, impairing the accuracy and sensitivity of analysis.